The airborne transmission of disease organisms, especially respiratory disease organisms has long been recognized as a serious problem in health care. With a growing population of immune deficient individuals due to the contraction of acquired immune deficiency syndrome (AIDS) and other debilitating conditions which weaken the immune system, the control of airborne disease transmission has become increasingly important and difficult. Air purification is perceived to be the only practical method of controlling airborne disease transmission. The increasing incidence of contraction of tuberculosis, pneumonia and other airborne disease in modern health institutions indicates, however, that known air purification systems are inadequate in controlling the spread of airborne microorganisms.
Air purification by means of filtration and irradiation are widely practiced. Traditional air treatment systems are commonly arranged in an order of filtration, irradiation and humidification. Irradiation is placed after filtration because the ultraviolet lamps used for that purpose readily attract dust which can accumulate on a surface of the lamp and interfere with their germicidal effect by absorbing and/or reflecting radiant energy. Irradiation is placed before humidification because ultraviolet radiation is most effective in a relatively dry atmosphere which promotes oxidation.
The germicidal effects of light rays of short wavelength has been recognized for more than half a century. A diverse array of methods and apparatus have been invented for irradiating fluids, and air in particular, in order to control the spread of microorganisms by destroying those microorganisms suspended in the fluid. The ultraviolet radiation for it's germicidal effect:
______________________________________ 2,070,307 - Nicholls 3,757,496 - Sievers 2,248,618 - Fischer 4,017,736 - Ross 2,279,810 - Arnott 4,694,179 - Lew et al. 2,628,083 - Rense 4,750,917 - Fujii 3,518,046 - Circirello 4,806,768 - Keutenedjian 3,576,593 - Circirello ______________________________________
Ultraviolet radiation has been proven to be more effective and economically feasible than any other approach to reducing the density of airborne microorganisms in an enclosed space. Two principle methods of utilizing ultraviolet radiation to destroy airborne microorganisms include air duct irradiation and the direct irradiation of the upper air in living or working areas. Although several studies have demonstrated convincingly that ultraviolet radiation can effectively reduce the density of airborne microorganisms, in practice wide variations in effectiveness have been documented.
Traditionally, air purification systems have relied on exposure of airborne microorganisms to ultraviolet radiation by passing air over or around one or more ultraviolet lamps. All of the patent references listed above relate to some variation on this method. The method has two principle shortcomings. First, exposure time depends almost exclusively on the rate of air flow around the lamps. Second, it is well known that ultraviolet radiation is readily adsorbed by most surfaces. As a result, accumulations of dust and particulates on radiant lamps adversely affects their germicidal effectiveness. Since the operation of radiant lamps generates an electrostatic field, they readily attract and accumulate particulate matter when placed directly in a flow of air, especially if the flow of air is unfiltered or poorly filtered. These factors may account in part for the variable results experienced to date with the use of ultraviolet irradiation in controlling the density of airborne microorganisms.
Although most microorganisms, including bacteria and viruses, are readily destroyed by sufficient exposure to ultraviolet radiation, the duration of exposure required to destroy a microorganism depends on a number of variable factors including humidity, the particle density in the air being treated and distance of a microorganism from a source of radiation.
Several important factors have been largely ignored in prior art disclosures for purifying air by destroying airborne microorganisms using ultraviolet radiation. It is well known that the strength of radiation decreases inversely with the square of the distance from a radiation source. Ultraviolet radiation is therefore most effective at close range. Relatively long exposure times may be required to destroy certain microorganisms, especially in humid environments. Host importantly, in order to ensure adequate radiation exposure microorganisms are preferably trapped on a filter surface before or during exposure, thus ensuring radiation levels and exposure times which are adequate to effect their destruction. Finally, radiation lamps must be protected from accumulating airborne particulate matter which can reflect and/or absorb radiation.
U.S. Pat. No. 4,694,179 entitled Symbiotic Filter-Sterilizer which issued Sep. 15, 1987 to Lew et al. teaches a cylindrical filter structure which surrounds, or is surrounded by, one or more ultraviolet lamps. The ultraviolet lamps are in turn encased in clear plastic tubes to protect the lamp(s) from fluids to be filtered. Although this invention appears to espouse some of the principles outlined above, it suffers from several shortcomings. First, the recommended filter is a porous filter through which most microorganisms pass unimpeded. Radiation exposure time is therefore random and dependent on the rate of fluid flow. Second, the transparent tubes which protect the lamps are exposed directly to fluids passed through the porous filter. The tubes therefore collect fine particulates which are resistant to ultraviolet radiation and absorb or reflect the radiation so that the effectiveness of the irradiation of the filter degrades with time. Third, the relationship between the fibers of the porous filter and the ultraviolet lamp is static. The fibers of the porous filter exposed to direct radiation therefore shade the areas of the filter not directly exposed. These "shaded" areas can provide pockets where microorganisms survive under the right conditions.
The prior art therefore suffers from several disadvantages which can be improved upon.